Wire mesh thermal radiative element and use in a radiative oven

ABSTRACT

A high speed cooking apparatus employing a low voltage high current system for heating foods employing a novel wire mesh heating element. The system herein described providing the benefits of high speed cooking like that further described by U.S. Provisional Application 60/822,028 filed on Aug. 10, 2006, but yet providing significant cost benefit and simplicity over said system.

U.S. Provisional Application 60/822,028 filed on Aug. 10, 2006 and co-pending patent application describe an oven capable of cooking foods at accelerated times compared to conventional ovens.

The following invention relates to the use of stored energy in combination with an infrared heating source consisting of a wire screen mesh element for the purpose of cooking or toasting.

BACKGROUND OF THE INVENTION

Specifically, the oven described consists of a stored energy system, a switching system, a food holder, and radiant heat bulbs used to cook the food. Typical cook times (in seconds) for a system running about 20 KW of power are described below:

Thin Slice Toast (white bread) 3.5 Bagel Half (plain) 5 Hog Dog (directly from refrigerator) 20 Pizza (directly from freezer) 22 Bacon Strips (grilled in fat) 30-40 Grilled Cheese Sandwich 10-15

The radiant heat bulbs are central to the prior art as they produce the appropriate wavelength of infrared energy required (in the range of 1 to 3 nanometers) and the multiple bulbs provide the intensity. Typical bulbs include halogen based bulbs similar to those produced by companies such as Ushio, Sylvania, or Soneko with power density of approximately 100 w/in2. Although these bulbs are effective at reducing cook times, they have several primary draw backs which have to this point deterred the prior art from successful introduction in the marketplace. Specifically;

-   1) The price for bulbs is high relative to the entire price required     to commercialize a unit such as a toaster. -   2) Bulbs can easily get damaged by oils and grease common in the     cooking process. -   3) Use of glass shielding over the bulbs decreases the intensity of     the radiant energy. -   4) Although fewer, longer, high voltage bulbs can be used, the     voltage poses safety risks and therefore, low voltages are     preferable. Unfortunately though, the use of smaller bulbs further     requires that many bulbs be used; complicating manufacturing and     overall pricing issues.

Another method for heating involves the use of Nichrome wire. Nichrome wire is commonly used in appliances such as hair dryers and toasters as well as used in embedded ceramic heaters. The wire has a high tensile strength and can easily operate at temperatures as high as 1250 degrees Celsius.

Nichrome has the following physical properties:

Material property Value Units Tensile Strength 2.8 × 10⁸  Pa Modulus of elasticity 2.2 × 10¹¹  Pa Specific gravity    8.4 None Density 8400 kg/m³ Melting point 1400 ° C. Electrical resistivity at room temperature 1.08 × 10^(−6[1]) Ω · m Specific heat  450 J/kg° C. Thermal conductivity    11.3 W/m/° C. Thermal expansion 14 × 10⁻⁶ m/m/° C. Standard ambient temperature and pressure used unless otherwise noted.

When considering the use of Nichrome within an oven it is important to consider not only the resistive characteristics but also the black body emission of the element when hot.

With regard to the general characterization of resistive elements,

The resistance is proportional to the length and resistivity, and inversely proportional to the area of the conductor.

$\begin{matrix} {R = {{\frac{L}{A} \cdot \rho} = {\frac{L}{A} \cdot {\rho_{0}\left( {{\alpha \; \left( {T - T_{0}} \right)} + 1} \right)}}}} & {{Eq}.\mspace{14mu} 1} \end{matrix}$

where ρ is the resistivity:

${\rho = \frac{1}{\sigma}};$

L is the length of the conductor, A is its cross-sectional area, T is its temperature, T₀ is a reference temperature (usually room temperature), ρ₀ is the resistivity at T₀, and α is the change in resistivity per unit of temperature as a percentage of ρ₀. In the above expression, it is assumed that L and A remain unchanged within the temperature range.

Also note that ρ₀ and α are constants that depend on the conductor being considered. For Nichrome, ρ₀ is the resistivity at 20 degrees C. or 1.10×10⁻⁶ and α=0.0004. From above, the increase in radius of a resistive element by a factor of two will decrease the resistance by a factor of four; the converse is also true.

Regarding the power dissipated from a resistive element, where, I is the current and R is the resistance in ohms, v is the voltage across the element, from Ohm's law it can be seen that, since v=iR,

P=i ² R

In the case of an element with a constant voltage electrical source, such as a battery, the current passing through the element is a function of its resistance. Replacing R from above, and using ohms law,

P=v ² /R=v ² A/ρ ₀ L  Eq. 2

In the case of a resistive element such as a nichrome wire the heat generated within the element quickly dissipates as radiation cooling the entire element.

Now, considering the blackbody characterization of the element:

Assuming the element behaves as a blackbody, the Stefan-Boltzmann equation characterizes the power dissipated as radiation:

W=σ·A·T ⁴  Eq.3

Further, the wavelength λ, for which the emission intensity is highest, is given by Wien's Law as:

$\begin{matrix} {\lambda_{\max} = \frac{b}{T}} & {{Eq}.\mspace{14mu} 4} \end{matrix}$

Where,

-   -   σ is the Stefan-Boltzmann constant of 5.670×10⁻⁸ W·m²·K⁻⁴ and,     -   b is the Wien's displacement constant of 2.897×10-3 m·K.

In an application such as a cooking oven, requiring a preferred operating wavelength of 2 microns (2×10E-6) for maximum efficiency, the temperature of the element based on Wein's Law should approach 1400 degrees K or 1127 degrees C. From the Stefan-Boltzmann equation, a small oven with two heating sides would have an operating surface area of approximately 4×0.25 m×0.25 m or 0.25 m2. Thus, W should approach 20,000 Watts for the oven.

In the case of creating a safe high power toaster or oven it is necessary for the system to operate at a low voltage of no more than 24 volts. Thus, using Eq. 2 with 20,000 W, the element will have a resistance of approximately 0.041 ohms, if 100% efficient at the operating temperature. Based on Eq. 1, a decrease in operating temperature to room temperature (from 1400 to 293 k) represents an approximate decrease in the resistivity of the element by about 1.44 times, and therefore an element whose resistance at room temperature is 0.0284 ohms is required.

Now, considering the relationship of the resistance of the element and the characterization of the element as a blackbody:

The ratio of the resistance of the heater to the black body radiative area of the same heater becomes the critical design constraint for the oven; herein termed the De Luca Element Ratio. The ideal oven for foods operating over a 0.25 square meter area at 2 micron wavelength has a De Luca Element Ratio (at room temperature), of 0.1137 ohms/m2 (0.0284 ohms/0.25 m2). The De Luca Element Ratio is dependant solely on the resistance of the material and the radiative surface area but is independent of the voltage the system is operated. In addition, for wire, the length of the wire will not change the ratio.

Table 1 lists the resistance per meter of several common nichrome wire sizes as well as the De Luca Element Ratio for these elements. It is important to note that all these wires have a De Luca Element Ratio far greater than the 0.1137 required for an oven operated at 1400K, 24V, and over 0.25 m2. Clearly the use of a single wire with a voltage placed from end-to-end in order to achieve the power requirement is not feasible.

In contrast, a household pop-toaster, operated at 120V and 1500 W, over a smaller 0.338 m2 area at 500K would require a De Luca Element Ratio of 35.5. Thus a 1 meter nichrome wire of 0.001 m radius with a 120V placed across it would work appropriately.

TABLE 1 Surface De Luca Time Resistance Area of Element To Reach Cross Per Meter 1 meter Weight Ratio 1400K Wire Radius Sectional Length length Per (at room At 20 kw (m) Area (m2) (ohms) (m2) Meter (g) temp) (sec) 0.01 3.14E−04 0.0034 0.0628 2637 0.1 65.4 0.0015 7.06E−06 0.15 0.00942 59.3 16.2 1.47 0.001 3.14E−06 0.30 .00628 26.3 47.7 0.654 .0005 7.85E−07 1.38 .00314 6.6 438 0.163 0.000191 1.139E−07  11.60 0.00120 0.957 9670 0.024 0.000127 5.064E−08  24.61 0.00079 0.425 30856 0.010 0.000022 1.551E−09  771.21 0.000138 0.013 5580486 0.0003

Clearly a lower resistance or a higher surface area is required to achieve a De Luca Element Ratio of close to 0.1137.

One way to achieve the De Luca Ratio of 0.1137 would be to use a large element of 2 cm radius. The problem with this relates to the inherent heat capacity of the element. Note from Table 1 that to raise the temperature to 1400K from room temperature would require 65.4 seconds and thus about 0.36 KWH of energy.

This calculation is derived from the equation relating heat energy to specific heat capacity, where the unit quantity is in terms of mass is:

ΔQ=mcΔT

where ΔQ is the heat energy put into or taken out, of the element (where P×time=ΔQ), m is the mass of the element, c is the specific heat capacity, and ΔT is the temperature differential where the initial temperature is subtracted from the final temperature.

Thus, the time required to heat the element would be extraordinarily long and not achieve the goal of quick cooking times.

Another way for lowering the resistance is to place multiple resistors in parallel. Kirkoff's law's predict the cumulative result of resistors placed in parallel.

The following Table 2 lists the number of conductors for each of the elements in Table 1, as derived using equation 5, that would need to be placed in parallel in order to achieve a De Luca Element Ratio of 0.1137. Clearly placing and distributing these elements evenly across the surface would be extremely difficult and impossible for manufacture. Also note that the required time to heat the combined mass of the elements to 1400K from room temperature at 20 KW for elements with a radius of greater than 0.0002 meters is too large with respect to an overall cooking time of several seconds.

TABLE 2 De Luca Number of Element Parallel Time To Ratio for Elements Reach single Required to 1400 K Wire element Achieve De Total At 20 kw Radius (@ Room Luca Ratio Weight/ (sec) From (m) Temp) of 0.1137 Meter (g) Room Temp 0.01 0.1 1 2637 65.4 0.0015 16.2 12 711 17.6 0.001 47.7 22 579 14.4 .0005 438 63 415 10.3 0.000191 9670 267 255 6.3 0.000127 30856 493 209 5.2 0.000022 5580486 6838 88 2.18

OBJECTS OF THE INVENTION

It is therefore an object of the current invention to:

-   1) Find a heating element capable of delivering the same power and     cooking characteristics as bulbs yet be significantly less     expensive. -   2) It is an object of the current invention that the heating element     have a temperature rise time of less than 2 seconds. -   3) It is further an object of the following invention that the heat     generated from the element be capable of being evenly distribution     over the cooking area. -   4) It is further an object of the current invention that the De Luca     Element Ratio, as defined herein, of the element be close to 0.11. -   5) It is also an object of the current invention that a resistive     nichrome element consist of an integral unit that is easy to     assemble into a unit such as an oven.

SUMMARY OF INVENTION

In summary, the following invention allows for the creation of a high power oven by using a resistive mesh element. The heater element designed so as to allow for the desired wavelength output by modifying both the thickness of the mesh as well as the surface area from which heat radiates. The heater consisting of a single unit mesh that is easily assembled into the oven and having a low mass so as to allow for a very quick heat-up (on the order of less than a few seconds).

Specifically, the wire mesh cloth design calibrated to have the correct De Luca Element Ratio for a fast response (less than 2 sec) oven application operating at 1400 degrees K.

To date, the best mesh design for operating a quick response time oven consisting of a nichrome wire mesh with strand diameter of 0.3 mm, and spacing between strands of 0.3 mm, and operating voltage of 24V.

DRAWINGS

The invention will now be further described in connection with the following graphs and photographs.

FIG. 1 is a graph illustrating the radiative area of a mesh element as a function of the center to center spacing of the mesh strands.

FIG. 2 is a graph illustrating the electrical resistance of a mesh element as a function of the radius of the strand and the mesh spacing.

FIG. 3 is a graph illustrating the ramp up time of a two sided 125 mm×250 mm mesh element oven as a function of the radius of the strand and the mesh spacing and power drain of 20 KW.

FIG. 4 is a composite graph of FIGS. 1 and 2, indicating the regions applicable for high speed oven cooking with a De Luca Element Ratio close to 0.11 ohms/m2.

FIG. 5 is a photograph of a small 24V oven built using the mesh system.

FIG. 6 is a photograph of a 0.3 mm×0.3 mm mesh using 0.3 mm diameter nichrome wire which operates well at 24V across a 200 mm oven.

DESCRIPTION OF DRAWINGS

In considering the best mesh design, it is important to evaluate the blackbody radiative area as well as the resistance of the element as a function of the following:

-   -   1) The number of strands per unit area of the mesh     -   2) The radius of the mesh strands     -   3) The mesh strand material     -   4) The potential for radiation occlusion between strands.

FIG. 1 describes the blackbody area as a function of the number of strands and the strand spacing of the mesh. Interestingly, the surface area is independent of the radius of the wire strand if the spacing is made a function of the radius.

Using equation 5 from above, the resistance of the mesh can be calculated for a specific wire strand radius. FIG. 2 illustrates the electrical resistance of a nichrome mesh element as a function of the radius of the strand and the mesh spacing. Limitation in Equation 5 become apparent as the number of strands becomes very high and the resistance becomes very low; thus atomic effects associated with random movement of electrons in the metal at room temperature form a minimum resistive threshold.

Using nichrome as the strand material in the mesh and operating the system at 20 KW, the ramp up time to achieve an operating temperature of 1400 degrees K is a function of the strand radius and the mesh spacing (note that a nominal mesh size of two times 125 mm×250 mm is used). FIG. 3 illustrates the region below which a ramp up of less than 2 seconds is achievable (note that wire radius above 0.5 mm are not shown due to the long required ramp up times).

FIG. 4 is a composite graph of FIGS. 1 and 2, indicating the regions applicable for high speed oven cooking with a De Luca Element Ratio close to 0.11 ohms/m2.

FIG. 5 is a photograph of oven 3 with top and bottom wire mesh elements 1 and 2 each 125 mm×230 mm and operated at 24V. Each wire mesh (1 and 2) has 766-125 mm long filaments woven across 416-230 mm long elements, each element 0.3 mm in diameter. A 24 V battery source is placed across the length of the 766 elements at bus bars 4 and 5. The wire surface area for a single strand of 0.14 mm diameter wire is 0.000440 m2/m. Thus, a total surface area (for combined top and bottom elements) can be calculated as:

Total Blackbody Radiating Area=2×0.000440×(416×0.23+766×0.125)=0.168 m2

The resistance across bus bars 4 and 5 as well as 6 and 7 was measured at 0.04+/−0.01 ohms. (Note that bars 4 and 6 as well as 5 and 7 are connected by cross bars 8 and 9 respectively.) Thus calculating the De Luca Element Ratio for the elements gives:

0.02 ohms+/−0.01 ohms/0.168m2=0.119+/−0.06 ohms/m2

which is within experimental error to the desired value for the De Luca Element Ratio providing the most optimal cook time. These experimental values also match closely to the expected values shown in FIG. 4.

Panels 10 and 11 are reflectors used to help focus the radiation towards the item placed in area 12.

FIG. 6 is close up photograph of the wire mesh 1 from FIG. 5, Mesh 1 is a 0.3 mm×0.3 mm mesh (2×R) using 0.14 mm diameter nichrome wire and operates well at 24V. Caliper 20 has a spacing between ends 21 and 22 of 2.0 mm for reference, bounding approximately 7 strands (spacing of 0.3 mm between strands). 

1-48. (canceled)
 49. A radiant heater comprising: a heating element having a radiative black body area; and a power supply to supply power to the heating element, wherein the radiative black body area is converted to be equivalent to 0.25 m² and a ratio of a resistance of the heating element to the equivalent area is less than 2 ohms/m².
 50. The radiant heater of claim 48, wherein the radiative black body area is over 0.25 m².
 51. The radiant heater of claim 48, wherein the radiative black body area is less than 0.25 m².
 52. The radiant heater of claim 48, wherein the ratio is approximately 0.1137 ohms/m².
 53. The radiant heater of claim 48, wherein the heating element comprises a wire.
 54. The radiant heater of claim 48, wherein the heating element has a specific heat capacity of less than 0.36 KWH of energy for raising the temperature of the heat element from room temperature to about 1400° K, wherein an equation relating a heat energy to the specific heat capacity, where the unit quantity is in terms of mass is: ΔQ=mcΔT where ΔQ is the heat energy put into or taken out of the element (where P×time=ΔQ), m is the mass of the element, c is the specific heat capacity, and ΔT is the temperature differential where the initial temperature is subtracted from the final temperature.
 55. The radiant heater of claim 48, further comprising a relay for cycling the power to the heating element, and a control circuit for controlling the relay.
 56. The radiant heater of claim 48, wherein the heating element comprises multiple heating elements, the multiple heating elements share a first bus and a second bus, the first bus is in electrical communication with a positive voltage of the power supply and the second bus is in electrical communication with a negative voltage of the power supply.
 57. The radiant heater of claim 48, further comprising a control circuit configured to monitor a condition of a load by measuring one or more of: a color of the load, a moisture level of a surface of the load, and a moisture level of air in the oven.
 58. The radiant heater of claim 48, further comprising a heating cavity configured to receive a load to be heated, wherein the heating element is sized and positioned to heat the load.
 59. The radiant heater of claim 48, wherein the power supply comprises a low voltage high current system.
 60. The radiant heater of claim 48, wherein the heating element has a combined resistance of less than 10 ohms.
 61. The radiant heater of claim 48, wherein the heating element comprises a wire having a thickness less than or equal to 1 mm.
 62. The radiant heater of claim 48, wherein the heating element comprises a wire mesh cloth.
 63. A heating method comprising: heating a heating element; and discharging current from a power supply through the heating element, wherein the radiative black body area is converted to be equivalent to 0.25 m² and a ratio of a resistance of the heating element to the equivalent area of the heating element is less than 2 ohms/m².
 64. The heating method of claim 63, wherein the radiative black body area is over 0.25 m².
 65. The heating method of claim 63, wherein the radiative black body area is less than 0.25 m².
 66. The heating method of claim 63, further comprising cycling the power to the heating element.
 67. The heating method of claim 63, further comprising monitoring a condition of a load by measuring one or more of: a color of the load, a moisture level of a surface of the load, and a moisture level of air in the oven.
 68. The heating method of claim 63, wherein the heating element has a combined resistance of less than 10 ohms. 